This invention relates to a method of enhancing the acid activity of certain high silica-containing crystalline materials by a process which involves contacting them with aluminum chloride (AlCl.sub.3) vapors, followed by hydrolysis and calcination.
High silica-containing zeolites are well known in the art, and it is generally accepted that the ion exchange capacity of a crystalline alumino-silicate is directly dependent upon its aluminum content. Thus, for example, the more aluminum there is in a crystalline structure, the more cations are required to balance the electronegativity thereof, and when such cations are of the acidic type such as hydrogen, they impart tremendous catalytic activity to the crystalline material. On the other hand, high silica-containing zeolites having little or substantially no aluminum have many important properties and characteristics and a high degree of structural stability such that they have become candidates for use in various processes including catalytic processes. Materials of this type are known in the art and include high silica-containing aluminosilicates such as ZSM-5 (U.S. Pat. No. 3,702,886), ZSM-11 (U.S. Pat. No. 3,709,979), and zeolite ZSM-12 (U.S. Pat. No. 3,832,449) to mention a few.
The silica-to-alumina ratio of a given zeolite is often variable; for example, zeolite X can be synthesized with silica-to-alumina ratio of from 2 to 3; zeolite Y from 3 to about 6. In some zeolites, the upper limit of silica-to-alumina ratio was virtually unbounded. ZSM-5 is one such example wherein the silica-to-alumina ratio is at least 5. U.S. Pat. No. 3,941,871 discloses a crystalline metal organosilicate essentially free of aluminum and exhibiting an x-ray diffraction pattern characteristic of ZSM-5 type aluminosilicates. U.S. Pat. Nos. 4,061,724; 4,073,865; and 4,104,294 describe microporous crystalline silicas or organosilicates wherein the aluminum content present is at impurity levels.
Because of the extremely low aluminum content of these silica-containing zeolites, their ion exchange capacity is not as great as materials with a higher aluminum content. Therefore, when these materials are contacted with an acidic solution and thereafter are processed in a conventional manner, they are not as catalytically active as their higher aluminum-containing counterparts.
The novel process of this invention permits the preparation of certain high silica-containing materials which have all the desirable properties inherently possessed by such high silica materials and, yet, have an acid activity which heretofore has only been possible to be achieved by materials having a higher aluminum content in their "as synthesized" form.